1. Field of the Invention
The present invention relates in general to air bag occupant supplemental restraint systems. In particular, the present invention relates to improved inflation gas flow arrangements for pyrotechnic air bag inflators of the type having two walls.
2. Description of the Related Art
Pyrotechnic air bag systems of various designs are well known. In general, such systems include an inflator which has a housing surrounding a mass of gas generant material. To initiate the gas generation, the inflator also includes a mass of initiating material and an initiator. An appropriate electrical signal will cause combustion of the initiator, or squib, which in turn causes combustion of the initiator material. The hot gasses created by the initiator impinge upon the gas generant, causing the generant to form the larger quantity of inflation gas.
This inflation gas exits the inflator and flows into the cushion of the air bag system. Continued gas flow causes the cushion to inflate and provide a compliant protective surface for the passenger. This process, from receipt of the signal to full inflation, typically takes on the order of fifty to one hundred milliseconds.
The need to produce a sufficient quantity of gas within the very short time period necessarily requires a relatively large gas pressure. This pressure results in several problems.
First, the gas exits from the inflator with a sufficient pressure that it may physically propel the entire inflator, in a manner similar to a rocket, if the inflator is not securely mounted. To prevent this, it has been known to direct the exiting gas in radial directions about the periphery of the inflator (known as a radial inflator). This results in the thrust from radially opposed gas jets cancelling out. Such designs are referred to as thrust-neutral, and are preferred.
Second, the gas generant produces a quantity of solid material during its activation. Due to the high pressure, the gas acquires a sufficiently high velocity to entrain this solid material within the gas flow. Since this solid material would be dangerous if it exited the inflator and struck the passenger, it must be sufficiently filtered. Filtration efficiency is reduced, however, by the high velocity of the gas. This is especially true in recent low-cost radial (thrust-neutral) designs.
In such designs, known as two walled designs, a first circular wall defines a central chamber for the initiator. Outside of this first wall the gas generant is placed in an annular chamber and is directly surrounded by the filter media. The filter media is in turn directly surrounded by a second, outer wall of the inflator, which includes the gas exit ports. While this design reduces the size and weight of the inflator, the gas enters directly into the filter with full velocity, reducing the efficiency of the filter.